The present invention relates to apparatuses and methods for the completion of mineral production wells. In particular, the invention is related to a perforating and gravel packing system and method.
Modern oil and gas wells are typically equipped with a protective casing which is run into the wellbore. Production tubing is then run into the casing for producing minerals from the well. Adjacent the production zones, the protective casing is perforated to allow production fluids to enter the casing bore. Since particles of sand are typically carried with the mineral from the production zone into the casing, it is sometimes necessary to install a gravel pack or production screen to filter the particles of sand. Therefore, it is common practice to complete a mineral well in two steps: (1) run-in the well with a perforating gun to perforate the casing; and (2) run-in the well with a gravel pack tool to gravel pack and/or isolate the perforated zone. However, this method is disadvantageous because it requires multiple trips into the well to perforate and gravel pack the zone.
To reduce the required number of trips into the wellbore casing, various single trip perforation/gravel packing devices have been developed. For example, as described in U.S. Pat. No. 4,372,384, incorporated herein by reference, a single trip apparatus for completing a formation in a case borehole is disclosed. The patent teaches the use of a tool string which includes a perforating gun, gravel packing tools and a packer means. The casing is perforated by running a gun firing device down through the tubing string. The well is allowed to flow freely to clean up the perforated formation. The system is then moved to position a sandscreen of the gravel packer adjacent the perforations and packers are used at each end of the screen to straddle and pack off the perforated pay zone. With the screen and packers in position, a gravel pack is established in the annulus between the perforated casing and the screen. The tool screen is left downhole in the casing as a permanent completion device. The produced fluid is allowed to flow through the perforations, the gravel, screen, and finally up through the tubing screen to the surface.
An alternative well completion system is disclosed in U.S. Pat. No. 5,954,133, incorporated herein by reference. In particular, a method of displacing a perforating gun in a well bore is used to perforate multiple zones without the need to unset or reset a packer. Multiple perforating guns in a positioning device are configured in an axially compressed configuration. The perforating guns are attached to the positioning device and inserted into the wellbore. With a first perforating gun positioned adjacent a first zone, the gun is fired to perforate the casing. The positioning device is then extended to axially displace a second perforating gun within the casing to a position adjacent a second zone. The second gun is then fired to perforate the casing. After a zone(s) has been perforated, the positioning device is further axially extended to displace a production screen and packer. The production screen is positioned adjacent the perforations and the packer is positioned opposite the perforations.
As illustrated in the above referenced patent documents, in traditional one-trip systems, the perforating gun assembly is mechanically connected to the gravel pack assembly during run-in and perforating operations. A basic problem with traditional one-trip perforation/gravel packing systems is that the gravel packing portions of the system are damaged when the guns of the perforation portion of the system are detonated. In particular, a major factor affecting the reliability of one-trip perforation/gravel packing systems is the effects of gunshock on the gravel pack assembly. This shock loading can be in the form of a mechanical force which is communicated through a pipe string or similar structure correcting the perforating guns to the gravel packing assembly. Alternatively, a pressure wave created during detonation in the fluid column inside the wellbore casing can damage the gravel packing apparatus due to a shock effect It has been very difficult to predict the size of this shock effect and even more difficult to prevent it.
Therefore, there is a need for a one-trip perforation/gravel packing system which is more reliable than traditional systems in that the gravel packing portion of the system is protected from gunshock generated by the guns of the perforating portion of the system.